AQA A-Level Chemistry Paper 1, 2017: Question 6

✅ Correct answer (what to write)

Mark scheme allows: 4P + 5O₂ → P₄O₁₀ and other correct multiples. State symbols are ignored.

💡 Key knowledge

• “Excess oxygen” means the highest oxidation state oxide forms: phosphorus(V) oxide, P₄O₁₀.
• Phosphorus commonly exists as P₄, so equations often start with P₄.

❌ Common errors (how marks are lost)

• Writing P₄O₆ (that’s with limited oxygen, phosphorus(III) oxide).
• Incorrect balancing of O₂ (e.g. 3O₂ instead of 5O₂).

🧠 Exam technique

• Spot “excess oxygen” → choose the most oxygen-rich oxide.
• Balance oxygen last: product has 10 O atoms → need 5 O₂.

✅ Model full-mark method (matches mark scheme points)

1. Add water to a small amount of each solid in separate test tubes and shake to make solutions.
2. Test each solution with litmus or universal indicator (or measure pH with a pH meter).
3. Expected results:
   • Sodium oxide forms sodium hydroxide solution → alkaline: blue litmus stays blue / red litmus turns blue; UI turns blue/purple; pH high (≈ 12–14).
   • P₄O₁₀ reacts with water to form phosphoric acid solution → acidic: blue litmus turns red; UI turns red; pH low (≈ 1–2).

✅ Correct answers

• Silicon dioxide (SiO₂): giant covalent (macromolecular).
• Sulfur trioxide (SO₃): molecular (simple molecular).

💡 Key knowledge

• Giant covalent = continuous network of covalent bonds throughout the structure (no discrete molecules).
• Simple molecular = discrete molecules with intermolecular forces between them.

❌ Common errors

• Writing “simple covalent” for SO₃ (explicitly not allowed).
• Calling SiO₂ “ionic” because it contains a metal/metalloid and oxygen.

🧠 Exam technique

• For structures, answer with the classification the mark scheme wants: “giant covalent” vs “molecular”.
• Don’t drift into bonding within the molecule unless asked (that’s for the next part).

✅ Full-mark explanation (linked to marking points)

• SiO₂ has strong covalent bonds between atoms in a giant covalent structure.
• SO₃ is molecular, with van der Waals forces (intermolecular forces) between molecules.
• Covalent bonds are stronger than van der Waals forces.
• So more energy is needed to break/overcome the covalent bonds in SiO₂ than to overcome intermolecular forces in SO₃ → SiO₂ has a higher melting point.

💡 Key knowledge

• Melting involves overcoming the forces that hold particles in the solid structure.
• Giant covalent solids: melting requires breaking many covalent bonds in a network.
• Simple molecular solids: melting only overcomes intermolecular forces; covalent bonds within molecules stay intact.

🧠 Examiner commentary (typical pitfalls)

• If you say “covalent bonds between molecules in SiO₂”, you lose the mark for M1 (SiO₂ is a network; there aren’t separate molecules).
• If you claim hydrogen bonding in SO₃, you lose the IMF mark (no H present).
• If you write only “SiO₂ has stronger bonds” with no mention of what forces in each, you cap your marks—this question rewards named interactions + comparison.
• Mark scheme warning: mentioning “ionic” or “metallic” bonding typically limits you to a maximum of 1.

❌ Common errors

• Confusing “bonding within SO₃ molecule” (covalent) with “forces between molecules” (van der Waals).
• Saying “SiO₂ has a higher melting point because it has more atoms” (not a mark point here).

✅ Correct equation

Also allowed by mark scheme: SO₃ + KOH → KHSO₄ (acid salt) or ionic versions (e.g. SO₃ + 2OH⁻ → SO₄²⁻ + H₂O ). Multiples allowed; ignore state symbols.

💡 Key knowledge

• SO₃ is an acidic oxide (acid anhydride of H₂SO₄).
• Acidic oxide + alkali → salt + water (or acid salt if alkali not in excess).

❌ Common errors

• Forgetting the water product when forming K₂SO₄.
• Incorrect potassium count (needs 2KOH to make K₂SO₄).

🧠 Exam technique

• Decide which salt you’re making:
  • Fully neutralised: sulfate (SO₄²⁻) → K₂SO₄.
  • Part-neutralised: hydrogensulfate (HSO₄⁻) → KHSO₄.
• Balance K first: sulfate needs 2K.

✅ Correct equation

Multiples allowed; state symbols ignored.

💡 Key knowledge

• Metal oxide + acid → salt + water.
• Phosphoric acid is triprotic (H₃PO₄) and forms phosphate salts (PO₄³⁻).

🧠 Exam technique (quick balancing shortcut)

❌ Common errors

• Producing Mg₃PO₄ (missing brackets/incorrect formula).
• Not balancing water correctly (check O and H atoms at the end).

✅ What you must draw

The acid is sulfurous acid, H₂SO₃. Draw sulfur in the centre with:

• One S=O double bond
• Two S–O–H single-bonded hydroxyl groups

Mark scheme note: ignore lone pairs.

💡 Key knowledge

• SO₂ + H₂O forms H₂SO₃ (undissociated acid for the displayed formula request).
• Displayed formula must show connectivity: which atoms are bonded to which.

🧠 Exam technique

• “Displayed formula” = show all bonds clearly. Put S central; attach O atoms; put H on O (not on S).
• Use one double bond to O, and two –OH groups to match H₂SO₃.

❌ Common errors

• Drawing H attached directly to S.
• Drawing the sulfate-like structure (too many O atoms) or H₂SO₄ (wrong acid).
• Forgetting to show both O–H bonds (then it isn’t H₂SO₃).

🧠 30-second self-check

• Equations balanced? (atoms conserved, especially O₂ and H₂O)
• “Excess oxygen” and “excess MgO” interpreted correctly?
• Structure terms correct: giant covalent vs molecular (avoid “simple covalent”).
• Melting point explanation names the right interactions and compares them.
• Displayed formula: H on O, correct number of O atoms.

💡 High-yield facts to remember

• Na₂O + H₂O → 2NaOH (alkaline)
• P₄O₁₀ + 6H₂O → 4H₃PO₄ (acidic)
• SiO₂ = giant covalent network; SO₃ = simple molecular with weak intermolecular forces
• SO₃ + 2KOH → K₂SO₄ + H₂O
• 3MgO + 2H₃PO₄ → Mg₃(PO₄)₂ + 3H₂O
• SO₂ + H₂O → H₂SO₃ (structure HO–S(=O)–OH)